This invention relates to the overheat preventing system of an A.C. motor.
When the number of poles or the frequency of a supplying power source of an A.C. motor is switched to operate the A.C. motor, an overheat is produced in the A.C. motor if the switching is not proper. This will be described in the below.
FIG. 1(a) shows a wiring diagram of a pole change motor. In FIG. 1(a), reference characters V.sub.R, V.sub.S, V.sub.T denote R-phase voltage, S-phase voltage, T-phase voltage, respectively of a three-phase A.C. power source, reference numerals 1, 2, 3 denote switches, reference numeral 4 denotes a pole change motor, reference numerals 4a, 4b, 4c denote the coils of the pole change motor 4, reference characters U.sub.1, V.sub.1, W.sub.1 denote the terminals of the coils 4a, 4b, 4c, reference characters U.sub.2, V.sub.2, W.sub.2 denote the intermediate terminals of the coils 4a, 4b, 4c, and reference numeral 0 denote the neutral point of the Y-connected coils 4a, 4b, 4c.
FIG. 2(a) shows an example of a conventional system for driving a fan by using a pole change motor. In FIG. 2(a), reference numeral 5 designates a fan, reference numeral designates 7 an air duct, reference characters 7a, 7b designate an inlet and an outlet, respectively of the air duct 7, reference numeral 8 designates an inlet vane for regulating the wind volume of the air duct 7, reference numeral 9 designates a wind volume command signal required by a system, and reference numeral 10 designates a controller for operating the opening of the inlet vane 8 upon receiving of the wind volume command signal 9. FIG. 2(b) shows a wiring diagram of the control circuit of the abovementioned system. Reference numerals 11, 12 designate control power sources, reference characters 1a, 2a designate contacts which become ON when the switches 1, 2 are closed, reference characters 1b, 2b, 3b designate contacts which becomes ON when the switches 1, 2, 3 are open, reference characters 1c, 2c, 3c designate operating coils (for closing the switches 1, 2, 3 when energized) of the switches 1, 2, 3, and reference characters PB.sub.H, PB.sub.L designate push-button switches which produce high and low speed operation commands, respectively of the motor.
The operation of the motor will be then described. The states of the switches at high and low speed times are shown in FIG. 1(b).
The voltages V.sub.R, V.sub.S, V.sub.T of the three-phase A.C. power source are applied to the coil terminals U.sub.1, V.sub.1, W.sub.1, respectively of the pole change motor 4 at the low speed time, and the coils 4a, 4b, 4c are connected at the neutral point 0 in a Y-connection.
On the other hand, the voltages V.sub.R, V.sub.S, V.sub.T of the three-phase power source are applied to the intermediate terminals U.sub.2, V.sub.2, W.sub.2, respectively of the coils 4a, 4b, 4c at the high speed time to form a Y-connection with the neutral point 0 as a center and a Y-connection with the neutral point formed by the closure of the switch 3.
Thus, the intermediate terminals are provided in the windings of the motor to form a pole change motor by switching the currents flowed to the coils.
In FIG. 2(a), showing an example of the conventional system for driving the fan 5 by the above pole change motor 4, the air is fed by the fan 5 from the inlet 7a of the air duct 7 to the outlet 7b. The wind volume passing the air duct 7 is controlled by applying the wind volume command signal 9 from the system to the controller 10 which converts the signal to a value adapted to control the inlet vane 8 and controls the opening of the inlet vane 8.
Heretofore, the switching of the pole number of the pole change motor 4 has been performed manually in the following sequence.
When the push-button switches PB.sub.H1, PB.sub.H2 are now pressed at the low speed time, the coil 1c becomes no voltage by the opening of the switch PB.sub.H1 to open the switch 1, and the contact 1b is simultaneously closed. Since the push-button switch PB.sub.H2 for bypassing the contact 2a is closed, the coil 2c and then the coil 3c are energized to close the switches 2, 3, thereby switching the motor to the high speed operation. The push-button switch PB.sub.H2 is reset when the switch is released, but the high speed operation is continued by the closure of the contact 2a. On the contrary, when the push-button switches PB.sub.L1, PB.sub.L2 are pressed at the high speed time, the coils 2c, 3c become no voltage by the opening of the push-button switch PB.sub.L1 to close the contacts 2b, 3b. Since the push-button switch PB.sub.L2 for bypassing the contact 1a is closed the voltages of the control power sources 11, 12 are applied to the coil 1c to close the switch 1, thereby switching the motor to the low speed operation. The push-button switch PB.sub.L2 is reset when the switch is released, but the low speed operation is continued by the contact 1a.
Since the operating system of the conventional pole change motor is constructed to be switched manually as described above, if the switching of the motor to the high speed is delayed, or motor is not switched to high speed operation due to a careless operator, the output required may be greater than the output of the motor at low speed. Thus, the conventional motor has such disadvantages that the operator must always take a notice for the operation of the conventional pole change motor. Further, since the switching of the motor to low speed operation may be delayed when becoming the output required is less than the output of the motor at the high speed, conventional pole change motor has such a disadvantage that the effect of electric power energy-saving cannot be performed.
Then, FIG. 3 shows a prior-art variable frequency power source system. In FIG. 3, reference numeral 51 designates a commercial power source (hereinafter referred to as "C power source"), reference numeral 52 designates a variable frequency power source (hereinafter referred to as "V power source"), reference numerals designate 53, 54, 55 switches, reference numeral 56 designates a motor selectively energized from the C power source 51 or the V power source 52, reference numeral 57 designates a rotor driven by the motor, reference numeral 58 designates a fluid passage, reference characters 58a, 58b designate an inlet and an outlet, respectively of the fluid passage 58, reference numeral 59 designates a control mechanism for controlling the fluid flow rate passing the fluid passage 58, reference numeral 60 designates a fluid controller, reference numeral 61 designates a control input to the controller 60, reference character 60a designates a converter for converting the control input into a control signal adapted for the number of revolution of the V power source 52 and the control mechanism 59, reference characters 60b, 60f, 60g designate contacts for opening or closing the control signal, and reference character 60e designates a signal generator.
Then, the operation of the conventional variable frequency power source system will be described. FIG. 3 will be described as an example of a fan for a boiler of a generating plant for the convenience of readily understandable description. In this case, the rotor 57 is a fan, the control mechanism 59 is an inlet vane, and the fluid passage 58 is an air duct.
When the V power source is operating, the switches 54, 55 are closed, and the switch 53 is opened. The motor 56 is energized by the V power source 52, and driven. The output of the V power source 52 is variable in the frequency F, and the number N of revolution of the motor 56 becomes as represented by the following equation: ##EQU1## The motor 56 is operated in the frequency F at the variable speed. The voltage E.sub.M (namely, the output voltage of the V power source 52) of the motor 56 is constant as E.sub.M =constant so as to avoid the saturation of the core of the motor 56 as represented by the following equation: EQU E.sub.M /F=K (Equation 2)
Thus the motor 56 is in general operated.
The fan 57 is driven by the motor 56. Therefore, the wind volume flowed to the air duct 58 is controlled by the number N of revolution of the motor 56, namely, the output frequency F of the V power source 52. At this time, the inlet vane 59 is necessary to reduce the resistance of the air duct 58 to perform the electric power energy-saving, and maintained in the constant opening in the full opening state or in the vicinity of full opening state.
Here, during the V power source operation, the contacts 60b, 60f are closed, and the contact 60g is opened. The control input 61 of wind volume is converted by the converter 60a into a signal adapted for the V power source 52, applied to the V power source 52 through the contact 60b, thereby controlling the frequency F of the V power source 52 to become the desired wind volume.
During the V power source operation, the inlet vane 59 receives the constant vane opening signal from the signal generator 60e through the contact 60f, thereby maintaining the constant opening in the vicinity of full opening.
On the other hand, at the C power source operation time the switch 53 is closed, and the switches 54, 55 are opened. The motor 56 is driven by the C power source 51, namely by the constant commercial power source frequency Fc, and accordingly the fan 57 is rotated at the constant speed. In this case, the fluid in the air duct 58 is controlled by the inlet vane 59.
More particularly, at the C power source operation time, the contacts 60b, 60f are opened, and the contact 60g is closed. The control input 61 of air flow rate is converted by the converter 60a into a signal adapted for controlling the inlet vane 59, and applied to the inlet vane 59 through the contact 60g. Thus, the opening of the inlet vane 59 is controlled to establish the desired wind volume.
The switching of the V power source 52 to the C power source 51 is achieved by opening the switches 54, 55, opening the contacts 60b, 60f and closing the switch 53 and closing the contact 60g.
When the V power source 52 must supply electric power to a large-capacity motor, the power source is not constructed to supply the maximum output of the motor so as to economically manufacture the power source, but is frequently fabricated in the capacity of the degree capable of supplying the electric power in an ordinary operation. In the case of the example of this fan, when the capacity of the V power source is formed to x % of the commercial frequency Fc in the operating frequency limit, it can become the value substantially proportional to (x/100).sup.3.
For example, when the power factor cos .theta.=85% and the motor efficiency .mu.= is 90% in the motor of 1000 kW, if the motor is intended to operate in its capacity of 1000 kW with the V power source, the power source requires approximately for the following capacity: ##EQU2## but if the ordinary operation of the motor is in a range of x=80% of the commercial frequency Fc, the capacity of the V power source can be selected to the following capacity: ##EQU3##
Therefore, the switching from the V power source to the commercial power source 51 can be performed during the V power source operation. Heretofore, the abovementioned switching operation has been manually carried out on the basis of the manual judgement.
More specifically, since the switching of the V power source to the C power source has been heretofore performed manually according to the operator's judgement, this had various drawbacks and disadvantages that it is always necessary to pay an attention that the motor has been operated within the capacity of the V power source and there was a danger of burning out the V power source due to the overheat by the overload of the V power source due to the operator's careless mistake and the necessary output could not be always attained.